One of the central problems in cell biology and medicine relates to the inability to monitor protein, lipids, sugars and metabolite levels and their modifications in the single living cell. A variety of technologies have been employed to improve the sensitivity of detecting these molecules.
For example, to increase the sensitivity of immunoassays able to detect proteins at very low amounts, the polymerase chain reaction (PCR) technology has been combined with conventional immuno-detection methods ((U.S. Pat. No. 5,665,539). This technology, termed immuno-PCR, provides an extremely sensitive method to detect proteins. In immuno-PCR, a linker molecule with bi-specific binding affinity for DNA and antibody is used to attach a marker DNA molecule specifically to an antigen-antibody complex, thus resulting in the formation of a specific antigen-antibody-DNA conjugate. The attached marker DNA can be amplified by PCR with the appropriate primers. As described in U.S. Pat. No. 5,665,539, antigen is immobilized on the surface of microtiter plates and subsequently detected by immuno-PCR. Using this technique, an approximately 105 increase in sensitivity over an alkaline phosphatase conjugated ELISA was obtained. Sensitivity advantages of immuno-PCR have subsequently been confirmed in assays for mouse anti-lipoprotein IgG (Ruzicka et al. Science 1993 260:698-699); a human proto-oncogene protein (Zhou et al. Nucleic Acid Res. 1993 21:6038-6039); and tumor necrosis factor alpha (Sanna et al. Proc. Natl. Acad. Sci. 1995 92:272-275).
However, the original immuno-PCR protocol used a streptavidin-protein A chimera to detect the antigen-antibody complex. The variation in the affinity of protein A against different classes of IgGs limits its direct application in the detection of a broad range of antigens. Certain improved protocols tried to solve this problem by introducing biotinylated secondary antibody or free streptavidins.
Joerger et al. (Clin. Chem. 1995 41(9): 1371-1377) demonstrated that double-stranded DNA labels can be directly attached to antibodies, thus allowing conjugate reagents to be prepared before the assay.
Suzuki et al. (Jpn. J. Cancer Res. 1995 86:885-889) describe a method called double determinant immuno-polymerase chain reaction (double-determinant immuno-PCR) which utilizes two monoclonal antibodies, in which the antigens are sandwiched, and a specific DNA molecule is used as a marker. In this method, the first monoclonal antibody to bind the circulating antigen is immobilized instead of the antigen itself. A biotinylated second monoclonal antibody is bound to the antigen and free streptavidin is used to attach a biotinylated DNA to the second monoclonal antibody. The biotinylated DNA complexed with antigen-antibody-streptavidin is amplified by PCR. The products are then analyzed by Southern blot analysis.
While these immuno-PCR techniques have provided advantages over traditional methods of protein detection such as an increase in sensitivity, there still exist several notable limitations to their use. One of the major limitations of immuno-PCR lies in the non-linear amplification ability of PCR reaction. There is no direct correlation between the amount of signal and the amount of protein present. Thus, this technique is limited as a quantitative detection method.
U.S. Pat. No. 5,922,553 discloses a method for quantifying levels of a selected protein via a technique referred to as immuno-aRNA. In this method, a first antibody targeted to a selected protein is immobilized to a solid support. The support is then contacted with the selected protein so that the selected protein is immobilized to the first antibody. The solid support is then contacted with a RNA promoter-driven cDNA sequence covalently coupled to a second antibody targeted to the selected protein so that the second antibody binds to the bound selected protein. The amount of selected protein is determined by quantifying levels of the promoter driven cDNA sequence covalently coupled to the bound second antibody via an amplified RNA technique. In a preferred embodiment, a T7 promoter driven cDNA sequence is covalently coupled to the second antibody.
It has now been found that single chain fragments as well as exocyclic peptide based complementarity determining region (CDR) subunits can be used in this immuno-aRNA technique. Further, it has been found that PCR, as well as amplified RNA techniques, can be used to quantify the promoter driven cDNA sequence covalently coupled to the bound single chain fragment or CDR subunit. The use of smaller antibody binding units and fragments coupled with the already existing large single chain or cyclic peptide libraries and the use of robotic assistance renders this method widely useful for both medicinal and research purposes. Furthermore, a single third detector species can be coupled with double-stranded DNA and bound to either the single chain Fv or the CDRs, rendering detection uniform and simple. This is referred to herein as a universal detector.